Method and apparatus for determining the direction of the electric axes of crystal quartz



1. H. DAWSON 1,866,454 RATUS FOR DE INING THE DIRECTION July 5, 1932.

METHOD AND APPA TERM OF THE ELECTRIC AXES OF CRYSTAL QUARTZ Filed Dec. 3, 1928 4 Sheets-Sheet l Insula ciorz Me taL f JI- Dawdon July 5, 1932. 1 DAWSON 1,866,454 I METHOD AND APPARATUS FOR DETERMINING THE DIRECTION OF THE ELECTRIC AXES OF CRYSTAL QUARTZ 4 Sheets-Sheet 2 Filed Dec. 3, I928 July 5, 1932. 1 DAWSON 1,866,454

METHOD AND APPARATUS FOR DETERMINING THE DIRECTION OF THE ELECTRIC AXES OF CRYSTAL QUARTZ Filed Dec. 3. 1928 4 Sheets-Sheet 3 July 5, 1932. 1 H, DAWSON 1,866,454

METHOD AND APPARATUS FOR DETERMINING THE DIRECTION OF THE ELECTRIC AXES OF CRYSTAL QUARTZ Filed Dec. 3, 1928 4 Sheets-Sheet 4 iib emor: 1760 H. flaumoro Patented July 5, i932 UNITED, TA ES P TENT OFFICE- i m H. DLWSCN, OF WASHINGTON, DISTRICT OF COLUMBIA, ASSIGNOR, BY KESNE ASSIGNMENTS, TO WIRED BADIOpINCJ, OF NEW YORK, N. Y, A CQBPORLTION OF DELAWARE maon AND arraaarus' roa nnrnammmc THE nmEc'rIoN or mu morale o axns or caxs'rar. eunarz Application nled December 8,1928. Serial 11o. 328,548.

invention relates broadly to an improved method and apparatus for determining the direction ofthe electric axes of crystal'quartz and more particularly to an electric method and apparatus for determining the direction of the electric axes of crystal quartz as distinguished from av visual examination of the crystallographic features of quartz. l

One of the objects of my invention is to provide a. method of electrically testing qua'rtz'preparatory to the cutting of piezoelectric plates from-the quartz.

Another object of my invention is to provide a method and apparatus for determining the piezoelectric properties of quartz and facilitating the cutting of the quartzinto P plates with a high degree of precisionwith respect to the piezoelectric axes of the quartz.

Still another object of my invention is to provide amethod and apparatus for cutting quartz with respect to the piezoelectric axes thereof, wherein a visual examination of the crystallographic features of the quartz is unnecessary for the accurate cutting of piezoelectric plates from the quartz.

Other and further objects of my invention are directed to the novel method and construction of apparatus which I employ for the preparation of piezoelectric crystals upon a quantity-basis with extreme accuracy as described more fully in the following specification by reference to the accompanying drawings, wherein:

Figure 1 shows diagrammatically the structure of a piezoelectric crystal; Fig. 2 is a side elevation of the quartz testing apparatus of my invention; Fig. 2a shows a portion of the apparatus of my invention which is employed for exerting pressure on the crystal;

. Figs. 3 to 9 show charts and diagrams of piezoelectric crystals showing the phenomena observed in carrying out my invention and Fig. -1O shows schematically the apparatus for determining the electrical axes of quartz crystals of my invention.

My inventionon the method of preparing quartz crystal is the result of an investigation perhaps more extensive than has been hitherto attempted of the piezoelectric efiect phenomena.

symmetry namely,

in crystalline quartz. The general laws of piezoelectric action, already well established, have been corroborated in full by the investigation but in addition many new and unsuspected facts have been discovered. The theory of the piezoelectric eflect available at present, although adequate for a description of the more general laws, is apparently incapable of coping successfully with the new It appears that" the formula tion of a; complete theory must await a more comprehensive understanding of the molecular structure of quartz. Further, the piezoelectric effect at various temperatures has been measured and in this connection experiments on the pyro-electric efi'ect have been erformed. At the outset, it will be understood that a crystal of quartz is in the form of a hexagonal the crystal, which may vary in length and breadth. lie at definite angles with each other. In Fig. 1, ,IEGK, GDLK, DLMF, MNHF, HNIC and CIJ E are the sides of the hexagonal cylinder, the angle between any two adjacent sides is 120. faces of the hexagonal yra-mid JHJMNIA- lie at an angle of 51 4 1 to the faces ofthe cylinder. The quartz crystal has four axes of Fig. 1. AB is called in crystallogra hie terms, the trigonal symmetrical positions of the crystal at it is rotated about this axis. 'AB is also called the optic axis, because along this direction cylinder surmounted by a hexagonal pyramid; the-faces of The- AB, on, an and en,

axis, as there exists t rec I the crystal has unique optical properties. CD, EF and GH are diagonal axes of symmetry, forthere are only two positions of symmetry as the crystal is rotated around each of these-axes. CD, EF and .GH are known as the electric axes, since a pressure applied to the crystal parallel to these direc- Physik und Chemie, NF19-20, 1883, 513, and direct attention to the more quantitative investigations of P. and J. Curie published in Comptes Rendu-XCI, 1880, in which were was directly proportional to the force, a posi-v tive charge accumulating on one of the aces and an equal negative charge on the opposite face. A reversal of the sign of the force produced a reversal of the charges. The magnitude of the charge, in c. g. s. electrostatic units, per square centimeter per. dyne of force normal to the surface was found to be 6.32 x 10--. esu/cm x dyne. This has been the accepted value. My investigations show that this value is not a constant for all quartz but varies very materially for different specimens of optically perfect uartz. This is in keeping with the general i eas of crystal imperfections as discussed later.

Pyroelectricity, which refers to the production of char e on the surface of a crystalline substance when heated has been regarded as really a piezoelectric effect arising from strains produced by unequal expansion with temperature, although there has been much controversy on the matter. There has been contributed a new and important view, that the piezo and pyroelectric phenomena are fundamentally different, and that the connection between themjs not simple. This is borne out in a genera'l wway in the present work. From extensive study of the mole cular structure of crystal quartz it may be concluded that pyroelectricty is due to the change of structure towards or away from hexagonal symmetry and that piezoelectricity is due to mechanical distortionof the uniaxial nature.

The apparatus of my invention is. shown in Fig. 2 comprising a-sensitive electrometer -.1 with a sensitivity of about .004 V/mm. suitably shielded by an earthed brass cylinder 2. The quartz 3 under investigation is placed upon the circular platform 4. This platform is fixed in a horizontal position in such a manner that it can be rotated about the vertical axis 5 and the amount of rotation measured by a circular scale on member 6. A longitudinal motion may be given the platform 4 by the screw 7 and measured by the divided head 8. Thus an point of'the quartz may be brought under t e copper contact point 9. The contact point is connected to the electrometer through lead 10 and insulated throughout by fused quartz. The apparatus is thoroughly shielded electrically and switches are placed in the system. Ingsome ofmy experiments, it was found desirable to appl or release the pressure on the quartz in a irection parallel to the plane of the platform 4. To accom lish this an instrument Fig. 2a is emplo ed in which a quick application" or release 0 pressure to the contact point is made by a suitably actuated cam 11. The

pressure applying bar 31 is mounted for 1on-' tudinal movement on flanged rollers 32 ournaled in the support 33, the weight 34 acting through lever 35 which engages notches in bar 31 to longitudinally force bar 31 to impart movement to contact 9a against the side of the piezoelectric crystal. To prevent physical displacement of the piezoelectric crystal on latform 4, an adjustable abutment36 is insu atingly mounted on m'em- The capacity '0 the system which varied during the investigation from 34 m. m. f. to 41 m. m. f. was compared with the capacity of a standard condenser by the heterodyne beat method, the measurementsbeing reliable to less than 1%. I

E; eriments were undertaken to determine the c aracter of the? distribution of charge over the surface upon which the pressure was applied. Specimens of optically perfect qkiliartz, were selected andcut in such a manner t at the electric axis was normal to the large face of a para'llelopiped 25 x 28 x 1 mm. Each crystal was placedon take 4, Fig. 2, and explored over the largest faces by releasing 100 rams pressure from the contact point whic had anarea of about mm.

In general one side of the crystal would give positive deflections while the opposite side gave negative deflections. Small areas of negative charge were often found on the faces which ave positive deflections over most of the ace; and corresponding areas of positive deflections were found on the negative. face. Large deflections might occur 'at the center of the quartz or at the edge and the deflection for one crystal was often twice that of another or stal. There seemed to be no uniformity 0 results. 1

The question arose whether such a distribution was of permanent character. To answer this a crystal was subjected to 'about 25 kilo-v grams pressure for 20 seconds andthe surface explored; no change in the distribution could be observed. The same crystal was then raised to a temperature of approximately 600 (1, thus transforming it to the ,B-quartz and after allowing it to returnto the a-quartz III state, the surface was again explored. 'Again no change was observed. Thus the distr1bu tion seems to be permanent. i

To substantiate further the above results, a piece of quartz cut in the above manner and about 150 mm. x 100 mm. x 3 mm. was subjected to the exploring test and again the charge was found to vary over the surface. The piece was then cut into three parts and each part explored, but no change could be observed in the character of the distribution of the original charge. Each part was ex-- amined optically and observed to be free from twinning and other defects.

These variations in the piezoelectric charge are naturally to be attributed .toimperfections in the quartz crystal, although these imperfections may be so minute as to escape detection by the usual examination with polarized light. All that can be said is that the imperfections might be in the nature of small crystal fragments variously oriented inperhaps a random manner with respect to the large parent crystal, and that these little fragments are smallin size, say less than 0.1 mm. in their largest dimension. It may be pointed out further thatthese imperfections may be very small, indeed such as would result from a displacement of a small group of molecules. Recent developments in X-ray analysis of crystals lead to the idea that crystals are not perfectly formed as hitherto supposed, but are constituted of a mosaic of more elementary perfect structures and that the axes of the more perfect crystals are not oriented in the same direction but may vary from a meanposition. This explanation may account for the peculiar variation of the piezoelectric charge over the surface of quartz.-

A cylindrical specimen of quartz mm. in diameter and 30 mm. thick was cut from a rough piezo of quartz free from flaws and twinning and the apex of the crystal re moved. An exploration of the distribution of the charge over the cylindrical surface was carried out by the application of apressure of' 1000 gms. directly to the exploring point which was placed perpendicular to the surface of the quartz. Care was taken to fasten the quartz cylinder concentrically .on table 4, Fig. 2 and to have points of reference marked on it in such' a manner that the data could always be referred back to the original specimen. The characterof the distribution is shown in Fig. 3 where the abscissac are the angles of rotation of the quartz which were read every degree and the ordinates areelectrometer deflections.

There appears to be one well defined region of positive charge and another of'negatii charge, the magnitude of the deflections of each being about equal, but the size of the positive region is larger than that of the negative.

cylinder. The exploration of the in the positive area and a similar phenomenon the region of point B in the negative area. It may be mentioned that the general characteristics of the curve always remained the same regardless of the method of holding the specimen. to the plate.

Next a cylindrical crystal of the same taking place in dimensions but cut as indicated in Fig. i was placed concentrically on table 4. The results of a procedure similar to the previous case is shown in Fig. 4 and it can be seen that there are'two well defined regions, one of the positive charge and the other negative. The maximum deflection in both the positive and negative regions are of the same magnitude, but appear to occur about 150 apart and not 180. dividing these areas lies more or less in the direction of the optic axis.

Finally, a crystal of the same dimensions as in the two previous cases was cut from a rough crystal in such a manner that the optic axis was normal to the ends of the surface wascarried out in a manner similar to the two preceding cases, and the character of distribution of the charge is shown in Fig. 5. There are three positive maxima and three negative and they occur accurately 60 apart and are the same magnitude and altermate in sign. There are six points where apart.

The above experiment serves as a new and of the electric axes of a piece of quartz cut normal to the optic axis and'in which there are no indications ofthe crystal form. The rough piece of quartz first must be examined for the optic axis and then cut into slabs of the desired thickness in which the optic is normal to the plane of the slab. The slab in which the directions of. theelectric axes are to'be determined is placed on table 4 Fig. 2 and an exploration of the surface, more or less parallel to the optic axis, is carried out and the points of either maximum or minimum deflection observed. In most cases the latter is preferable as being more accurately determined. In case two diametrically oppo- The direction of the .line

cylindrical precise method for determining the directibn the charge becomes zero and they lie- 60 parallel to the optic axis. Thus placing the quartz on the table 4 in such a manner that thecenter of the notch coincides with the center of rotation of the table, the exploring point can be brought in normal contact with this surface.

Many trials were made with this procedure v of quartz samples in which faces were present and others in which faces were absent. In either case the electric axis could easily be determined within 2.

Samples of quartz 20 x x 2 mm. and 20 x 25 x 1 mm. were cut with an electric axis normal to the 28 x 25 side and the two faces in contact with the electrodes parallel to each other within .025 mm. Forces of 50, 100, 200,

500, 1000 and 2000 grams were applied to jtal. that more consistent results could be obtained each in the direction of the electric axis and in such a manner as 'to' be equaly distributed over the entire surface of the crys- It was found after a few experiments by sputtering the surfaces of the quartz in contact with the electrodes with platinum. Four samples of the results obtained are given in table 1 and are representative of the results obtained in many trials on different crystals.

Crystal Number 1 was first explored by the point method and it was found that while each surface produced charges of like character there appeared to be a variation of 250 per cent in the magnitude of the piezoelectric charge; No regularity of distribution could be detected on any of the crystals.

The crystal was then subjected to test for the production of piezoelectric charge over the entire surface. The resulting charge is given in Table 1, crystal 1a. The crystal was next cut into three ieces of unequal size and the piezoelectric cliarge of each measured with the resulting charge as given in b, 0, d

Table 1. The largest of these three pieces was cut into two pieces and the charge determined-e and 7 Table 1. It can be seen that the charge produced varies very markedly among the separate pieces and that the charge for the whole piece is smaller than that for any one. q

In order to show the accuracy with which the measurements could be repeated. crystal Number 2 was measured on different days and under as nearly identical conditions as possible. Both the positive and negative faces tested with the result that the charge on the positive face appeared to be uniformly smaller than that on the negative as is shown in crystals 2a, b, 0, cl, e, f, g of Table 1. The results taken for the different days on the negative side of the quartz agree with each other within the limit of error than one would expect in this type of experiment, and the.

same may be said of the positiveside.

As an example of the difference of charge produced on different samples of quartz, 3

and 4 of Table 1 are crystals cut from opt1- cally. perfect material in as nearly the same manner as ossible. They were subjected to test Within a very few minutes of each other thus insuring as nearly identical condi- Table 1 Dimension in mm. of Crystal number surface in Charge esu/cm'x dyne Temp.

contact with electrodes -a.27a:.2a) X10.'.- 19.5" o. --6.37i.11) x 10- 24 5 7.l8;!=.16)xl0' 24 6.893=.10) x 10- 24 :12 7.ll:!=.l6) x10- 21 (1') 11% x12% fi.43=l=.18) x10' 21 2 (a); 27 x26 (6.05=I=.1b)x10 (b) 27 x 25 6.8Rd:.l2)x10 (0)-- 6.16==.16) x10- 0.42i.30) x 10- E 4.9-1=e.40) x10- 5.38104) 1 10- 5.47i.26)x10- 4 27 x 25 6.4ld=.05) x 10- 24 C.

Fig. 2' and the electric furnace laced over it.

The apparatus was suitably shlelded electrically and the temperature measured by achromel nichrome thermo couple. The

charge was produced by lifting a 500 gm.

weight from the quartz. The temperature was varied by convenient intervals to a point well above 576 (1, thus passing the trans formation point of a-quartz to ,B-quartz. The apparatus was brought to the desired temperature and allowed to stand until temperature equilibrium conditions had been reached. Six readings were taken and the mean of the six was used as the recorded result. Two crystals from different pieces of quartz were used.

The curves in Fig. 6 exhibit the results, where the abscissze are the temperatures of the quartz and the ordinates are electrometer deflections. The upper curves were taken with ascending and the lower with descending temperatures. It is seen that'the curves for both crystals are of the same character, starting from approximately the same value at room temperature and rising to a maximum at about 60 C. From this maximum.

. one case and 480 C. in the other. Beyond thirds'in the the piezoelect Y v electricity, the oints o 7 sides approximately of similar nature there is a slow decrease to around 300 C. Beyond300 C. there is a v in the charge produced at a out 440? C. in

these points the effect is extremely small. In the neighborhood of 450 C. the effect has practically disappeared.

Upon cooling no deflectionof the'electrometer could be detected until the apparatus had cooled down to 280 (3., and then there appeared a very rapid rise to a maximum value at 60 C. and from here to room temperature a decreasingcharge was observed. The maximum reached on cooling was approximately one-half in one case and twoother of the maximum reached on heating, and the resulting piezoelectric effect at room temperature after cooling was,

about one-half of the value of that before subjecting the crystal to heating. It was found that after the apparatus had remained untouched for 24 hours that both crystals had returned to their previous piezoelectric condition. I

It was thought that the uliarities of the curve might be due to e oxidation of the copper electrode, although this was very slight, but on substituting an' alum num rod for copper the same results were obtained. ,There were no thermo electric efiects even when aluminum was used.

The maximum in the curve has no evident explanation and was not expected. The lag in To efiect, however, is a heex ted, since other e ects ave been observed in the transformation offl uartz to a-quartz.

In order to obtain rther information in regard to the pyro and piezoelectric produc tion of electric charges on crystal quartz, the following experiments were unde Until very recently experiments were conducted on the crystal structure of quartz,

oelectricity was thought to be a mani- Pl festation of piezoelectricity brought about nomenon to be by stresses in thecrystal due to heating and A p ate of quartz cut perpendicular to the o tic axis when heated will have a distributtion of electrical charges over the face perpendicularto the optic axis. The character of this distribution will be such that there are six alternate areas. of sitive and negative maximum charge coinciding with t e electric axes and points of zero charge coinciding. with lines lying at 30 to the electric axes. When this char e is removed from the heated crystal by a ame and the crystal allowed to cool a distribution of electricity of like character is produced but with the signs of the charges reversed.

A section of hexagonal quartz crystal with 20 mm. long and 7 mm. thick was cut per ndicular to the optic axis and-placed on Ta 1e 4, Fig. 2, and heattd to I 200 (3., the rapid decrease int rtaken.

nal stresses in vrious coefiicients of expansion 1n the different surface discharge with a flame and the quartzthen allowed to cool to room temperature. The surface was then explored with the point, care being taken to locate the position of the exploring point with respect to a convenient system of axes. The results of these measurements are given in Fig. 7 iii-which the numbers indicate the sign and relative magnitude of the charges. It can be seen that the hexagon can be marked off roughly into six areas, three with charges of positive sign and alternating with these are the other three areas with charges of negative sign. The maximum charges appear roughly along the electric axes with lines of zero charge roughly along lines at 30 to the electric axesy Pressure was applied uniformly by means of a circular clamp to the sides of a piece'of quartz, 33 mm. in diameter and 12 mm. in thickness, cut perpendicular to the optic axis. Wren the circular surface was explored no charges or at least extremely small charges, were with as great a force as possible and the surface again explored along the same'line. In

found. The clamp was then tightened this case there appeared to be a distribution 1 of charges of the same character as was produced by heating but much smaller in magnitude.

' Pressure was applied in the direction of the electric axis to ahexagonal piece of quartz about 20 mm. on a si cut so that the plane f the hexagon was per .pendicular to the optic axis. The surface per ndicular to-the optic axis was explored e usual manner. The results are shown in Fig. of rotation of the quartz from the zero position and the ordinates electrometer deflections. It is seen that the distribution of e and 7 mm. thick and 8 in which the abscissae are the angles charge is similar in character to that produced by heating or cooling as in Fig. 7.

When the pressure was applied to the above hexagonal of quartz in the plane of the sample but at an angle of 30 to an electric axis an entirely different distribution of charge was produced. As Fig. 9 shows, that instead of finding six regions of electric charge there were but two areas of approximately equal size.

According to the prevalent ideas, -pyro electricity is due simply to the production of piezoelectricity brought about by the interthequartz caused by the vaponent along the electric axis will always 3? ing, when minimum illustrated in Fig.

exist, But the two experiments described above show this not to be the case 5 therefore, t

it would appear that the distribution of electric charge over the surface brought about'by different phenomecooling.

The apparatus for determining the electrical axes of or al quartz is schematically of the accompanying drawings. The piece of quartz 3 to be investigated is placed in metallic plate 4. Me-

tallic plate 4 is provided with scales 4a and 46, so

20. Heating units 20 may ematically respresenting the adjustments shown in Fig. 1 of the accompanying drawings. Metal plate 4 is electrically insulated from arm 23. Arm 23 supports searching contact 9. Movable weight 22 adjustably positioned on arm23 schematically represents the pressure adjustment shown in Fig. 2 of the accompanying Searching contact 9 is electrical y to switch member 25a. Metallic plate 4 may be electrically connected to electrometer 27 or may havea common ground connection 24 therewith. .This schematically represents the pressure method. The temperature method comprises a receptacle 30 which may comprise six sides or any suitable construction desired. Contained in the base 4 and the sides of container '30 or in both the base and sides may be mounted heating units be energized by any convement source of energy 21. The crystal element 3 to be investigated is placed in receptacle 30 and explored with searching contact 9. I Searching contact 9 is electrical- 13 connected to switch member 256. Suitable adjusting mechanism for shifting position of searching contact9 could be employed as shown in Fig. 1. The sides of receptacle 30 are shown removed for clearness of illustration. Base 4 may be of metal and electrically connected to ground 24 or the entire receptacle 30 may be of metal and connected to ground 24. Switch arm 25 may contact with either switch member 25a or 256 depending upon whether the pressure or the temperature method is desired. Switch arm 25 is electrically connected to switch member 26. Switch member 26 ma be arranged to reverse the polarity depend ing upon the design of electrometer 27 and the connections to the quartz crystal 3 under examination. Shield 31 surrounding the connecting wire may be connected to ground I electrometer Searching contacts the surface of crystals 3 until maximum readings are obtained. The parts of. the crystal 3 upon which searching-members 9 are resting, when maximum readings of 27 are obtained, coincides with the electrical axes. The parts of the crystal 3 upon which searching members 9 are restreadings are obtained,

vention.

drawings. connected 9 are moved over coincide with lines lying at 30 to the elecrical axes.

a' paratus and method for determining the electrical axis of quartz crystals are ossible without de arting from the irit o my int is obvious that t e method and apparatus of any invention may be employed for investigating materials other than quartz and'while the investi ation of quartz crystals has been referred to in the foregolng specification it is to be understood that my invention is not limited thereby. I

It is further understood that many modifications differing from those described in the foregoing specification and illustrated in the accompanying drawings are ossible without departing from the spirit 0 my invention and it is not intended that the embodiments of my invention be restricted thereby but only as defined in the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows: v

1. The method for determining the electrical axes of crystals comprising the stc s of cutting a section from aid crystal t 0 plane from which is normal to the o tical axis, subjecting different areas of sai sec- Irealize that many modifications of the tion to a mechanical pressure and indicating on said crystal, whereby the axes of maximum piezoelectric effect may be located in said crystal.

3. The method of determining the axes of quartz crystals comprising, stressing said crystal and indicating the iezoelectric effeet at different ointson t e face of said e axes of maximum efl'ect crystal whereb t may be locate LEO H. DAWSON.

potentials existing 

